It is well known that an internal combustion engine having an expansion ratio larger than the compression ratio will have a higher thermal efficiency. An engine with such a feature was first taught by Atkinson and was termed as the Atkinson cycle. In an Atkinson cycle, combustion gas continues to expand within the cylinder to the atmospheric pressure. Various mechanisms aimed to realize the Atkinson cycle were proposed or developed. However, many of these Atkinson-cycle based engines may involve a complex mechanical structure and bulky size, which may result in a mechanical weakness.
An alternative to the Atkinson cycle is the Miller cycle through early or late closing of the intake valve to decrease the compression ratio. The Miller cycle differs from the Atkinson cycle in that the engine's structure remains the same as that of an engine operating on a conventional four-stroke cycle. With an engine under part load conditions, the Miller cycle may have the benefit of reducing the pumping losses by eliminating the charge throttling. One common practice of evaluating the performance of a Miller cycle is to consider its performance similar to the Atkinson cycle. In this evaluation, the Miller cycle's reduced compression ratio due to the early or late closing of the intake valve was used against the engine's expansion ratio, and it was claimed that the Miller cycle would have a thermal efficiency equivalent to that of the Atkinson cycle. According to this evaluation, a Miller cycle therefore would have a thermal efficiency higher than that of a conventional cycle, such as an Otto cycle or diesel cycle. This comparison may be confusing, however, because the calculation is based on the decreased compression ratio for both Miller and conventional cycles. It is well known that a larger compression ratio will provide a higher thermal efficiency. As a result, an engine operating on a conventional cycle without throttling, with its compression ratio being equal to the engine's full compression ratio, may in fact have a higher thermal efficiency than that operating on a Miller cycle.
Another technique that may substantially increase the engine's thermal efficiency is the variable compression ratio mechanism. It is well known that the compression ratio in an engine design is determined largely based on the knock threshold at the wide-open throttle condition. However, this knock threshold may not be applicable to a part throttle condition when an engine is under part-load conditions, which may allow a higher compression ratio to increase the engine's thermal efficiency. Since majority of an engine's operating time would occur under part load conditions, there is a strong incentive to enable a variable compression ratio mechanism in the engine design. Many techniques that may enable a variable compression ratio have been proposed or developed. These techniques may vary the compression ratio by moving crankshaft axis, varying the piston stroke, moving the cylinder head, varying the combustion chamber volume, modifying connecting rod geometry, moving the crankpin within the crankshaft, or varying the piston deck height. However, so far no engine implementing any of these variable compression ratio techniques mentioned above has reached the production level.
Yet another issue facing the internal combustion engine industry is the cold start problem particularly in connection with alternative fuels such as ethanol and methanol. Although ethanol and methanol are considered to be renewable and their utilization has been promoted as the fuel of future, an internal combustion engine operating on ethanol or methanol would normally encounter cold-start problems, due to their large latent heat and lower vaporization rate. Over the years, many techniques have been considered to overcome the cold start problem. These techniques include gasoline pilot start, onboard distillation systems, glow-plug ignited fuel systems, electric superchargers, quick-heating of intake manifold, liquid-heated fuel injector rails, and phase-changing catalysts. Although some of these techniques may be able to solve the cold start problem, they may have the disadvantages of increasing the complexity of the engine system, causing inconvenience of consumers, or taking a long period of time to start the engine.